This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Levonorgestrel (LNG)-releasing implants (e.g. Norplant) and LNG-releasing intrauterine devices (LNG-IUS;e.g. Mirena IUS) are effective methods of contraception for women. However, women using these methods frequently experience breakthrough uterine bleeding (BTB) that often leads them to discontinue this form of contraception. Rhesus macaques also display BTB when treated with LNG-IUS, and can provide an animal model for assessing therapies to control the bleeding. Our goal in this research is to utilize rhesus macaques to characterize the effects of LNG-IUS on the endometrium and to test intermittent progesterone antagonist (PA) therapy as a treatment for BTB triggered by LNG-IUS use.